This invention relates to a process for making screw fasteners and more particularly to a process for making plated, self-drilling screws where only selected portions of the screws have been hardened. The invention also relates to the product resulting from the selective hardening process.
There are a number of industrial applications where it is necessary to mount a relatively thin panel onto a member of an underlying frame. Among these are the mounting of body panels to the frame of an automobile, curtain walls to the structure of a building and paneling intended for use with large home appliances such as dishwashers and automatic washing machines. In these applications and many others, such mounting is accomplished with screw fasteners.
In many operations where screws are used as fasteners, it is a practice to pre-drill and thread pilot holes to receive them. Where the structure is simple and the surfaces are readily accessible, such a practice does not impose much of an economic penalty. However, where the structure is more complex as on an automobile assembly line, or involves field work, as in automobile body shop work or in the building trades, the necessary tooling can be quite complicated and using such tooling can impose considerable time and cost penalties.
To simplify such work, self-drilling, screws are often used. These screws have the tip area configured as a drill point so that they can drill and tap their own pilot holes, thus offering a considerable reduction in assembly time.
In order to perform the drilling and tapping function the cutting surfaces of the screws must be harder than the work material. Typically the screws have been formed of steel which is subsequently hardened. The hardening operation can be completed in any one of several standard production methods, e.g. nitriding in a furnace with a controlled atmosphere, cyanide hardening in pots and Shaker hearth and quench.
By far, the majority of thread forming fasteners are hardened in bulk in rotary furnaces with a controlled nitrogen atmosphere. This results in a fastener which is hardened over its entire surface even though only the drill tip and first few threads of the screw do all the actual cutting.
Many of the above applications for self drilling, self threading screws involve long-term exposure to corrosive environments, so the process for making ferrous metal screws, in particular, customarily includes the step of plating the screws with a corrosion resistant metal layer after the hardening operation. The corrosion resistant layer may include one or more layers of electrodeposited copper, nickel and/or chromium and the like.
Unfortunately, when hardened steel is electroplated with the corrosion resistant layer it becomes much more brittle through a process called "hydrogen embrittlement". A fastener which has been hydrogen embrittled has a greatly increased risk of cracking at the head or shank when driven or torqued during assembly. Soft steel, however, is not hydrogen embrittled during the plating process.
In order to relieve the hydrogen embrittlement and reduce the danger of cracking the fastener during installation, standard specifications call for the fastener to be heat treated for several hours at 400.degree. F. immediately after electroplating of the corrosion resistant layers. Such post-plating heat treatment is not always completely successful at relieving the hydrogen embrittlement. A more certain method of preventing hydrogen embrittlement is needed.